Outpatient normal volunteers ate 6 eggs daily and diets high in saturated fat. The plasma cholesterol concentration increased in five subjects during the first two weeks but had returned to pre-study levels when the study was terminated at the end of four weeks. In the high-density lipoproteins of all subjects, a subfraction changed in that it developed an increased content of the arginine-rich apoprotein (ARP). When the ARP content increased, the HDL was able to bind to cells in tissue culture and inhibit cellular cholesterol synthesis. HDL without ARP were not able to bind to high affinity binding sites.